Rumen degradation characteristics of ryegrass herbage and ryegrass silage are affected by interactions between stage of maturity and nitrogen fertilisation rate

The objective of this experiment was to evaluate interaction effects between stage of maturity and N fertilization rate on rumen degradation characteristics determined with nylon bag incubations of ryegrass herbages and ryegrass silage. Grass herbage (n = 4) was cut after 3 or 5 weeks of regrowth and received a low (20 kg N/ha) or a high (90 kg N/ha) fertilization rate. Grass silage (n = 6) received a low (65 kg N/ha) or high (150 kg N/ha) fertilization rate and was harvested at early (c 2000 kg DM/ha), mid (harvested 13 d later), or late (harvested 34 d later) maturity stage and ensiled in big bales. All grasses were incubated in the rumen of three lactating rumen-cannulated Holstein Friesian cows. Rumen degradation characteristics of organic matter (OM), nitrogen (N) and neutral detergent fibre (NDF) and the extent of effective degradation (ED) were evaluated. In grass herbage, NDF content varied between 390 and 454 g/kg DM and N content between 12.1 and 25.8 g/kg DM. In grass silage, NDF content varied between 438 and 593 g/kg DM and N content between 13.4 and 34.8 g/kg DM. In general, rumen degradation of grass herbage and grass silage decreased with increased maturity, and increased with increased fertilization rate. Significant interaction between maturity and fertilization rate was observed for ED of OM, N and NDF, except for ED of N in grass herbage. These results indicate that the effect of the rate of N fertilization on degradation of nutrients in the rumen of dairy cattle and on nutritional value depends on the grass maturity stage.